A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. A CD-ROM disc containing a computer program listing appendix is submitted and is herein incorporated by reference, CD-ROM contain a single text file created on Sep. 21, 2001 and format IBM-PC.
1. Field of the Invention
The present invention relates to medical imaging systems, and more particularly to an improved acquisition scheme for an electronic portal imaging system.
2. Description of the Related Art
The use of linear accelerators for medical therapy is well known. Such linear accelerators are used for treating patients with radiation therapy, such as X-rays or electron beams. Such X-rays are created when high energy electrons are decelerated in a target material such as tungsten. Alternatively, the electrons themselves may be used directly for treatment. The major modules in a linear accelerator are a gantry with a treatment pad, a stand, a control console and a treatment couch. The stand is anchored firmly to the floor and the gantry rotates on bearings in the stand. The operational accelerator structure housed in the gantry rotates about a horizontal axis fixed by the stand for treatment of a patient lying on the treatment couch.
In the radiation therapy treatment of a patient, geometric accuracy is an important factor to the success of the treatment. One known method for local control of the delivery of radiation is a record and verify system which includes an imaging section for delivering an image using a fluoroscopic technique and an image processing unit for digitally processing the image. In the imaging section, a fluorescent screen converts images generated by the X-rays, which are emitted from the treatment head and then passed through the patient, into visible images. The visible images are then reflected to a video camera by a reflector in order to avoid irradiating the camera. In the image processing section, the video signals from the camera are digitally processed in real time for continuous monitoring of the treatment field throughout the treatment.
After the linear accelerator is turned on, there is a delay period, referred to as the acquisition delay, before the actual radiation beam is turned on. The imaging device typically detects the linear accelerator being turned on and begins the process of radiation detection (the linear accelerator""s internal signal that ejects the electron beam is typically isolated from the imaging device). Radiation detection is accomplished by looping through the acquisition of several short duration test images and evaluating their intensity levels. Once the radiation levels exceed a predetermined threshold, the imaging device transitions to a process called automated gain control, to determine upper and lower bounds of a desired analog to digital convertor (ADC) window. During the automated gain control process, a test image referred to as the AGC image is acquired and analyzed, typically by a CPU remote from the imaging board, to identify the lower bound (LOB) of signals of interest (SOI) and the upper bound (UPB) of signals of interest. Once the ADC settings are calibrated they are downloaded to the imaging board along with other acquisition parameters. The imaging device then begins the acquisition of actual portal acquisition images.
The above described acquisition scheme suffers from several disadvantages. First, the radiation detection threshold is typically a predetermined hard coded value. Thus, radiation detection can be impaired by changes in camera sensitivity either through the aging of the electronics or through replacing one camera with another.
In addition, look-up table techniques are generally used to scale the image pixel values from 16-bit to 8-bit due to the nature of the imaging board and processor structure. The duration of a test image acquisition is 25% of that of a portal image acquisition. Accordingly, a first look-up table is typically employed for the test image and second look-up table is typically employed for the actual acquisition image. Because a new look-up table for the actual acquisition duration must be downloaded after the automated gain control setting of the ADC window, the video capturing operation must be turned off and be turned back on, a process which requires about one-half of one second. The use of the two look-up tables thus inserts an undesirable acquisition delay. Accordingly, it is desired to provide for image acquisition that minimizes or eliminates this delay.
The prior art also suffers from several disadvantages in identifying the LOB and UPB. As noted above, a test image is acquired to determine these bounds. The image has two or three segments: 1) the background with no radiation, 2) the treatment port where the radiation exists from the patient, and 3) the air in field where the radiation hits directly onto the detector screen. An air in field segment might not occur in every case. After the AGC image is acquired, its histogram is generated. The histogram, a probability versus intensity map, is used to define the lower bound between the background and the signal of interest. The upper bound or UPB is typically, by default, the maximum intensity level on the AGC image, unless xe2x80x9cair in fieldxe2x80x9d is manually identified by the user, in which case the UPB is empirically set to the maximum intensity minus 5% of the total intensity range. Because a user is required to identify whether or not an air in field situation exists, there exists a potential for errors in the determination of the bounds of the signal of interest. Accordingly, it is desirable to provide a method for calibrating the LOB and UPB automatically, and automatically determining a UPB regardless of whether an air in field situation exists.
In order to calibrate the LOB, imaging systems typically are required to transfer both the AGC image and its histogram from the imaging board to the host computer. This transfer time is typically on the order of about 200 milliseconds, another undesirable delay. Still others transfer only the AGC image. However, since the AGC image is much larger than the histogram, undesired delay can result. Accordingly, it is desirable to provide a method for calibrating the LOB and UPB while eliminating the requirement of transferring both the AGC image and its histogram. Additionally, it is desirable to provide such a method using only the histogram.
Finally, in typical prior imaging systems, the image acquisition duration is typically independent of the radiation duration. Thus, if an acquisition is completed before the radiation treatment is finished, the rest of the radiation is wasted. Accordingly, it is desirable to provide a method to continue the acquisition until after the linear accelerator is turned off to insure maximum imaging efficiency and possibly improving the image""s statistical quality.
Accordingly, there is provided an improved acquisition system and method for a medical imaging system. According to one aspect of the invention, adaptive radiation detection is provided. The imaging device acquires several test images during a preacquisition delay. The intensity of the test images is used to determine whether the radiation is turned on. The maximum intensity level from these test images is defined as the upper limit of a camera""s response in the darkness. The standard deviation of the intensity distribution is also computed. The image intensity level of the subsequent test images are compared to a radiation detection threshold which is determined to be the upper limit of the camera""s dark signal, plus twice the standard deviation of pixel intensity distribution of the dark test images. If the intensities of a statistically significant number of pixels exceeds the threshold, the radiation is considered to be on.
According to another aspect of the invention, a single look-up table is used for both the test images and the actual acquisition images. The single look-up table is downloaded upon detection of power to a linear accelerator. According to still another aspect of the present invention, calibration of a lower bound of a signal of interest occurs using a histogram-based algorithm, so that only the histogram needs to be transferred from the imaging board to the host computer. Furthermore, the present invention is capable of automatically calculating the upper bound of a signal of interest, without requiring a user to input whether an air-in-filed situation exists. In addition, the present invention is configured for continuous image integration such that each acquisition cycle is repeated until the linear accelerator is turned off. Thus, the final image is the integration of images from each acquisition cycle.
According to another embodiment of the present invention, the upper and lower bounds of a signal of interest are automatically determined from the histogram of one or more of a series of test images. More particularly, a first derivative of the histogram is calculated. The first derivative is used to determine a first approximation to the lower bound, which is defined to be the location of the largest negative drop in the first derivative. A first approximation to the upper bound is also determined, which is defined to be the location of the maximum intensity level. Successive local integrals are then calculated. The lower bound is revised upward from the preliminary lower bound based in part on a value of the local integrals. The upper bound is similarly revised downward, also based in part on a value of the local integrals.